Optimization Of Multi-leaf Collimator Design

Multileaf collimator(MLC) is becoming a standard accessory of modern linac in shaping radiation fields.It has been widely used in conformal radiotherapy(CRT) and intensity modulated radiotherapy(IMRT).After about 40 years' development,MLC basic structure is stabilized.The important characters of MLCs include leaf end shapes,leaf cross sections,leaf widths and leaf positioning machnism.One limitation of the MLC is the undulation pattern when using it to shape radiation fields.That means the radiation field shaped by MLC has a stepwise boundary,and is not identical to the desired field that exactly conforms to the target. There is always under-blocked and(or) over-blocked areas.To overcome this limitation,two kinds of optimized designs are proposed in this study.Firstly,we developed an optimization model for determining leaf widths for single layer MLC.The optimization model regards leaf widths as variables,and the total area of discrepancy between MLC shaped fields and desired fields as an objective function,and the total width of all leaves as a constraint.A problem described by the model is solved with the hybrid of a simulated annealing technique (ASA,Lester Ingber,1993) and a gradient technique(DONLP2,ESpellucci,2001). The performance of the optimization model was evaluated with 634 target fields continuously selected from the patient data base of a treatment planning system. Optimal leaf widths were obtained for an MLC with total leaf pairs of 28,40,and 60, respectively,which corresponded to three types of conventional MLCs.The optimal leaf width first decreases slightly and then superlinearly increases with the distance away from the central line.Compared with MLC with conventional leaf width arrangement,the MLC with optimal leaf widths reduced the total area of discrepancy by 11.1%,28.6%,and 25.0%,respectively.Secondly,we introduced a new leaf end design applicable to dual layer MLCs.An optimization model was first constructed to describe the problem of determining leaf end shapes,and the corresponding problems were then solved by the simplex search method or the simulated annealing technique.The optimization process was based on 634 target fields selected from the patient data base of a treatment planning system. The optimal leaf end design is determined to be a semicircle.With the arc leaf end design,the radiation field shaped by MLC has a smoother boundary rather than a stepwise boundary and the conformity of MLC shaped fields to the desired target fields is improved by 32.3%.This study introduced optimization techniques to the MLC design for the first time and proposed two kinds of new designs for MLCs.With the new designs,the conformity between MLC shaped fields and target fields is improved significantly without increasing the number of MLC leaf pairs.